Methods for controlling resources in a communication network

ABSTRACT

The present invention refers to the field of communication networks having a number of nodes connected to the same link, the capacity of the link being divided into frames which in turn are divided into time slots. A procedure is used for informing each node on the link of which nodes that are connected to the link and which slots that it has access to. A verifying procedure is used for subsequently verifying that information separately held by said nodes is not inconsistent regarding the nodes&#39; right to allow sending of data in said slots. In addition, said verifying procedure is disabled from producing a positive verification during transition periods at which different nodes on the link risk having inconsistent link data.

FIELD OF THE INVENTION

[0001] The invention refers in general to a method for controllingresources in a communication network having a number of nodes connectedto the same link, the capacity of the link being divided into frameswhich in turn are divided into time slots.

BACKGROUND OF THE INVENTION

[0002] In communication networks of the above mentioned kind, differentschemes are used for determining which nodes that should have access towhich slots, i.e. have the right to set up communication channels andstart sending data using said slots. Such schemes include those thatallow modification, over time, of which nodes that have access to whichslots, for example to adapt to different nodes varying need for transferresources or to adapt to the situation that nodes are added or removedfrom the link.

[0003] For example, WO9736402 discloses a communication network, whereindifferent nodes are assigned so-called ownership of different slots. Anode that owns a slot has the right to allocate the slot for use by oneof the communication channels handled by the node. This document alsosuggest that a slot that is owned by a first node may be temporarily“lent” to a second node, allowing the second node to temporarily use theslot for sending data, after which the second node “returns” the slot tothe owner thereof. Furthermore, ownership to the slot can be transferredfrom one node to another, thereby transferring the right to control howthe slot is to be used.

[0004] In networks of this and similar kind, it is vital to ensure thatno two nodes send data using the same slot over the same link segmentsimultaneously, thereby risking to compromise each others traffic. Thisis sometimes referred to as ensuring that the allocation of slots is“conflict-free”.

[0005] The object of the invention is to solve the problem of how toefficiently allow and perform dynamic alterations in the network whileat the same time ensuring conflict-free allocation of resources using asimple mechanism.

SUMMARY OF THE INVENTION

[0006] The object of the invention is achieved by the invention as setforth in the accompanying claims.

[0007] According to an embodiment of the invention, there is provided amethod of the above mentioned kind comprising: a procedure for informingeach node on the link of which nodes that are connected to the link andwhich slots that it has access to; a verifying procedure forsubsequently verifying that information separately held by said nodes isnot inconsistent regarding the nodes' right to allow sending of data insaid slots; and the step of disabling said verifying procedure fromproducing a positive verification during transition periods at whichdifferent nodes on the link risk having inconsistent link data.

[0008] Use of a verifying procedure for verifying, for example, that anode is free to use a specific slot may produce an erroneousverification when alterations are taking place on the link. This isespecially true for alterations affecting data being of a type thatdifferent nodes participating in said verifying procedure use as a basisfor determining their input to said verifying procedure. For example,when changes in the number of nodes that are connected to a link takeplace, or when changes in which node that controls allocation of whichslots take place, different nodes will gain knowledge of said change atdifferent points in time and may thus participate in a verificationprocedure based upon inconsistent view's of the status of the link.Advantageously, according to the invention, to avoid this resulting inerroneous verifications, the verification procedure is disabled duringsuch periods of change. And as the node relies on the verificationprocedure as a prerequisite for actually putting new slots into use, nosuch activities can be undertaken while the verification procedure isdisabled, thereby ensuring that no slot access conflict causes two nodesto start sending data into the same slot simultaneously.

[0009] Preferably, any node detecting the occurrence of such a type ofchange into new link state will disable the verification procedure untilit has received acknowledgement that other nodes are aware of the newlink state. If, as preferred, the verification procedure comprises onenode sending an verification request and expecting all other nodes onthe link to reply to said request, any node can advantageously,according to the invention, disable the verification procedure by simplynot sending replies to the verification request.

[0010] Thus, as soon as one node on the link becomes aware of the riskof inconsistency, for example by having its Link State Protocolreporting new data on which other nodes that are connected to the link,the other nodes will become indirectly aware of it since the node willnot respond to inquiries. This is very advantageous compared to thealternative where a warning message is sent from the interface thatbecomes aware of the risk of conflicts, since such warning message willalways run the risk of not reaching all of its intended recipients.

[0011] Thus, the method according to the invention ensures that asituation where there is a risk of inconsistency in the distribution ofaccess to time slots is recognized and dealt with fast so that thesituation where two interfaces consider themselves to have access to thesame time slot is prevented, especially during link state changes.

[0012] Furthermore, the use of the idea that no reply is a negativereply makes the method insensitive to loss of messages and thus reliablein a network where there are alterations.

[0013] Note however that alternative embodiments could allow nodes tosend replies during the periods of change, as long as such messagesimplies that slots are free for use. A node could for example continuesending verification replies, however indicating warnings or indicatingthat no slots should be put into use before all nodes have the same viewof the new link state.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] An exemplifying embodiment of the invention will now be describedwith reference to the accompanying drawings, in which:

[0015]FIG. 1a illustrates an exemplary network of the kind addressed bythe invention;

[0016]FIG. 1b illustrates an exemplary frame structure used in thenetwork of FIG. 1a; and

[0017]FIGS. 2, 3, and 4 are schematic signaling diagrams illustratingmessage-exchanges among the nodes of the network in FIG. 1a.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0018] An exemplary communication network NW of the kind addressed bythe invention is shown in FIG. 1a and comprises three nodes A, B, and Cthat each is connected to a single ring link L via respective linkinterfaces I. On the link L, a recurrent, essentially fixed size frameof the kind illustrated in FIG. 1b is transported uni-directionally.

[0019] In the exemplified network, the capacity of the link L is dividedinto frames, each having a nominal duration of 125 μs and is turn beingdivided into a fixed number of 64-bit time slots. The start of eachframe is identified by a so-called synchronization slot S, and the endof each frame is provided with so-called fill slots F included toaccommodate for small jitters in the network frame frequency. Theremaining slots of the frame are slots to be allocated used fortransporting control signaling and payload data, respectively, betweenthe link interfaces I connected to the link L. This type of frame formatis for example used in so-called DTM networks (DTM—Dynamic synchronousTransfer Mode).

[0020] The three nodes A, B, and C are assigned ownership of respectiveslots on the link L, i.e. to respective sets of slot positions withineach recurring frame on link L. Slot ownership is determined by a masternode on the link (said master being appointed as the node having, as faras the nodes are aware, the lowest link layer address on the link). Inthe figures, it is assumed that node A is the master node on the link.Any node having been assigned ownership of a slot by the master node mayallocate the slot to form part of a communication channel, or it maydecide to lend the slot to another node that for one reason or anotherrequests more resources. Note that the owner of a slot hence need notnecessarily have immediate write access to the slot, as the write accessto the slot may have been borrowed by another node on the link. Notealso that whereas the distribution of slot ownership is controlledcentralized by the master node, the decision to lend the actual writeaccess to the slot, as well as the obligation to initiate verificationswith respect to the slot, rests with the node that is the owner of theslot.

[0021] Management of access to slots is handled by control messages sentand received by the nodes of the network using predefined controlchannels, said messages including the following message types:

[0022] A Change (CH) message is sent by a master node on the link (saidmaster node being appointed as the node having the lowest link layeraddress on the link) to inform the nodes connected to the link ofchanges in which nodes are connected to the link and in the distributionof slot ownership among said nodes. The Change message comprises a listof the nodes connected to the link together with a respective scalarcorresponding to the time slots that the nodes are allocated ownershipof on the link. The Change message is forwarded to reach all nodes onthe link on a node-to-neighbor-node-basis until it the last node on thelink returns the message to the master node. If the master hasn'treceived the Change message within a predefined period of time, it willresend the Change message. A node that receives a Change message willonly store the information contained therein and forward the massage ifthe list of nodes identified therein is consistent with the list ofnodes that the node's own Link Status Protocol has determined for thesubject link. Otherwise, the node will discard the Change message andthus forward it to a next node.

[0023] An Ownership Request (OR) message is sent to the master node fromany node that receives a Change message and is unhappy with the amountof resources that it has been allocated in the Change message. TheOwnership Request message identifies the amount of slots that thesending node would like to have ownership of.

[0024] A Verification Request (VREQ) message is used when a node wantsto verify the write access situation with respect to one or more timeslots that it is the owner of. It is a message that is sent from theinvestigating node to all other nodes on the same link and identifiesthe slot (or slots) that the verification pertains to. A node willrepeatedly send Verification Request messages with respect to the slotsthat is the owner of. For slots that it has not lent to any other node,it will use the Verification Request message to verify that no othernode believes itself to have any access right to said slots. For slotsit has lent to other nodes, it will use the Verification Request messageto verify that said other nodes still regard themselves as borrowingsaid slots and that they hence, for example, haven't ended borrowing theslots without any message indicative thereof having been received by theowner of the slot. Moreover, for all new slots that a node is assignedownership of in a Change message, either during operation or at linkstart-up, it will send a Verification Request message to these nowslots, to verify that no other node regards itself as having access tosaid slots, such verification, received from all other nodes on thelink, being a prerequisite for the node to start allocating said slotsfor actual use and thereby allowing data to be sent in said slots.

[0025] A Verification Reply (VREP) message is used by each node as thereply to a received Verification Request message. It is a unicastedmessage that is sent from each node that has received a VerificationRequest message to the node that was the sender of that VerificationRequest message. It identifies whether or not the node sending theVerification Reply message considers itself as having access to the slot(or slots) that the Verification Request message pertained to, forexample by regarding itself to be the owner of the slot or to becurrently borrowing the slot.

[0026] A Resource Announce (RES_ANN) is sent by each node, telling allother nodes on the link how many slots it currently is willing to lendto other nodes. Other nodes that receive this message store thisinformation to know which node to ask for resources if in need to borrowsuch.

[0027] A Resource Request (RES_REQ) message is sent from a node thatneeds to borrow slots to the node that it tries to borrow slots from,and identifies the number of slots that it would like to borrow.

[0028] A Resource Transfer (RES_TR) message is used when a node lends(or returns after borrowing) access to a slot to another node. It issent from the node that transfers the slot access to the node thatreceives the node access and identifies the slot (or slots) for whichaccess is transferred.

[0029] An Interrupt (IN) message is sent by the node having theownership of a slot when detecting, via a verifying procedure, that twoor more nodes are accessing the same slot. The Interrupt message is sentto instruct these node to stop using the slot so that no more than onenode on the link has access to the slot.

[0030] In this exemplifying embodiment, each node will keep a state forall slots one the link. From the point of view of the individual node,each individual slot on the link will always occupy one (and one alone)of the following states:

[0031] FREE:

[0032] The node owns the slot and it has been verified that it is freefor the node to use. The slot can be allocated to a channel or lent toanother node as desired. If the node receives a Change messageindicating that it is no longer the owner of the slot, it will changethe status of the slot to GONE (se below). If the node receives anInterrupt message, implying that there is an access conflict andinstructing the node to stop using the slot, it will do so, and changethe state of the slot to LENT.

[0033] LENT:

[0034] The node owns the slot but has lent it to another node. When theslot is returned, the node will change the status of the slot to FREE.At repeated intervals, slots in the LENT state will be transferred tothe VERIFYING state (se below) to verify that they have not been “lost”but is in fact still used by other nodes. If the node receives a Changemessage indicating that it is no longer the owner of the slot, it willchange the status of the slot to GONE (se below). Also, at start-up, allslots owned by the node are considered to be lent until a verificationprocedure has verified that that is not the case.

[0035] GONE:

[0036] The node is not the owner of the slot and has not borrowed itfrom another node. If the node receives a Change message indicating thathas become the owner of the slot, it will change the status of the slotto LENT.

[0037] BORROWED:

[0038] The node is not the owner of the slot, but is using it whilehaving borrowed it from another node. When returning the slot to theowner after having borrowed it, the node will change the status of theslot to GONE. If the node receives a Change message indicating that hasbecome the owner of the slot, it will change the status of the slot toBUSY (se below). If the node receives an Interrupt message, implyingthat there is an access conflict and instructing the node to stop usingthe slot, it will do so, and change the state of the slot to GONE.

[0039] BUSY:

[0040] The node is the owner of the slot and is using the slot (i.e. ithas been actually been allocated to a communication channel). If thenode receives a Change message indicating that it is no longer the ownerof the slot, the node will change the status of the slot to LENT. If thenode receives an Interrupt message, implying that there is an accessconflict and instructing the node to stop using the slot, it will do so,and change the state of the slot to LENT.

[0041] VERIFYING:

[0042] The slot is undergoing an examination of whether some other nodeclaims any access to the slot or not. When the node has receivedVerification Replies from all other nodes on the link, indicating the noother node is using the slot or considers itself to have any otheraccess to the slot, the node will change the state of the slot to FREE.If the Verification Replies indicated that another node is using theslot, it will change the state of the slot to LENT. If the VerificationReplies indicate that two or more other nodes are using the slot, thenode will send Interrupt messages to all but one of these nodes toimmediately resolve the conflict. If the node receives a Change messageindicating that it is no longer the owner of the slot, the node willchange the status of the slot to GONE.

[0043] In an alternative embodiment in which different nodes are allowedto send data in the same slot position, but over separate segments ofthe link to ensure that data is not corrupted, the above set of statesis managed by the node on a per-slot-and-segment basis and not merely ona per-slot basis, as the slot, from the node's point of view, can be indifferent states for different segments of the link.

[0044] In addition to the above, each node keeps track on whether or notsending of Verification Request and Reply messages is currently allowed.If a change is currently taking place regarding which nodes that areconnected to the link or regarding the distribution of slot ownership,Verification Request and Reply messages are temporarily not allowed, asdescribed more specifically in the following.

[0045] In this exemplifying embodiment, a node will disable verificationby refraining from sending Verification Request and Verification Replymessages when: a) its Link State Protocol detects a change in whichnodes that are connected to the link; or b) the node receives a Changemessage identifying a list of nodes that is inconsistent with the listlast reported by the node's own Link State Protocol. The node willresume sending Verification Request messages and Verification Replymessages when it receives a Change message identifying a list of nodesthat is consistent with the list last reported by the nodes own LinkState Protocol.

[0046] Furthermore, a master node on the link will disable verificationby refraining from sending Verification Request and Verification Replymessages when its Link State Protocol reports a change in which nodesthat are connected to the link. It will then transmit a Change messagereflecting the new list of nodes connected to the link. The master nodewill subsequently resume sending Verification Request messages andVerification Reply messages when it again receives the Change messageafter having been forwarded among the nodes connected to the link.

[0047] The master node will also disable verification by refraining fromsending Verification Request and Verification Reply messages when it hasdecided to make changes in the distribution of slot ownership. It willthen transmit a Change message reflecting the new distribution ofownership. The master node will subsequently resume sending VerificationRequest messages and Verification Reply messages when it again receivesthe Change message after it having been forwarded among the nodesconnected to the link.

[0048] According to an alternative embodiment, a node will, when causedto disable its sending of Verification Requests and Replies with respecta link connected to one of interfaces, also disable its sending ofVerification Requests and Replies with respect other links that the nodehas interfaces to. The reason for this is that if a node is connectedto, for example, a dual ring, a detected change with respect to one ofthe rings can be regarded likely to be accompanied by a correspondingchange on the other one of the rings. As inconsistencies hence mightoccur on both rings, it can accordingly be regarded as most safe to forthe node to simply disable the verification procedure with respect toboth rings.

[0049] Schematic signaling diagrams illustrating message-exchanges amongthe nodes of the network in FIG. 1a will now be described with referenceto FIGS. 2, 3 and 4, illustrating the three nodes A, B and C with arrowsrepresenting messages transmitted among the nodes. Note that time flowsfrom top to bottom in the figures.

[0050] In FIG. 2, node A, being the master node, starts up the link bysending (at time t1) a Change message CH to node B, which in turnforwards the message to node C to then be returned to node A (at timet2). The change message identifies a list of the nodes connected to thelink and the distribution of ownership to slots on the link. As nodes Band C in this case is assumed to store link data (as derived by a LinkState Protocol not presented more in detail herein) that correlates withthe data included in the change message, they accept the Change messageand forward it as described. Otherwise, they would have discarded themessage, causing retransmission thereof from the master node.

[0051] During the time interval from t1 to t2, the master node willrefraining from sending Verification Request and Verification replymessages, thereby effectively disabling the verification procedure, asillustrated further below. To be noted, throughout these figures, thepatterned columns along the timelines of the different nodes, such asthe one between the markings t1 and t2 in FIG. 2, represent timeintervals during which the respective node is disabling the verificationprocedure by refraining from sending Verification Request andVerification reply messages.

[0052] At t3, node B starts sending Verification Request messages VREQwith respect to all the new nodes that it was assigned ownership of inthe previous Change message. As neither node A nor node B disable theverification procedure at this point in time, they will both reply tonode B with Verification Replies VREP messages, which are received atnode B at time t4. If we assume that neither of these replies indicatethat A or C is using the slots that the Verification Request referredto, node B may now regard these slots as free and may accordinglyallocate them for use as desired. To be noted, nodes A and C willperform similar verification procedures with respect to the slots thatthey were respectively assigned ownership of in the Change message CH.However, for simplified explanation, these parallel verificationprocedures have not been illustrated in the figure.

[0053] At t4, it is assumed that node A, for one reason or another, hasa need for more resources, and therefore sends a Resource Requestmessage RES_REQ to node B, asking to borrow a number of slots that nodeB is the owner of. Node B is assumed to be able to accommodate thisrequest, and therefore transfers access to a set of slots to node A bysending a Resource Transfer message RES_TR to node A. This message isreceived at node A at time t7, which may then start using the set ofslots, identified in the message, as borrowed slots.

[0054] At time t8, node A is finished using the borrowed slots and“returns” them to node B by sending a Resource Transfer message RES_TRto node B, identifying the subject set of slots. However, for unknownreasons, the message gets lots on its way to node B and is neverreceived at node B. Node B will therefore continue to believe that nodeA is still borrowing the slots.

[0055] However, node B will repeatedly perform verification procedureswith respect to all slots that it is the owner of and that it considersto be lent to other nodes. At the end of a timer local to node B, ittherefor sends, at time t9, a Verification Request message VREQreferring to the lent set of slots to all other nodes on the link. NodesA and C responds using Verification Reply messages VREP that reachesnode B at time t10. As non of these identify node A or node C as usingthe slot, node B can conclude that the slot is no longer borrowed bynode A (nor by any other node). Node B may therefore again consider thesubject set of slots as free for and may accordingly allocate them foruse as desired.

[0056] Continuing with reference to FIG. 3, at time t11, the master nodeA has determined a need for changing the distribution of ownership amongthe nodes connected to the link and therefore send, at t11, a new Changemessage CH identifying the new distribution to the other nodes on thelink. At the same time, it disables its participation in theverification procedure, as illustrated by the start of the patternedcolumn.

[0057] At t12, node B receives the Change message CH, identifies the newdistribution, and forwards the Change message CH to node C, whichsubsequently forwards it back to node A to reach node A at time t14.

[0058] It is assumed that the Change message CH states that node B hasbeen given ownership of more slots that previously. Node B willtherefore, immediately after receiving the Change message CH, initiateverifying procedures with respect to all new slots that is has becomethe owner of. This is done at time t12 by node B sending VerificationRequest messages VREQ to nodes A and C, identifying the new slots. NodeC replies to the Verification Request message it has received from nodeB by sending a Verification Reply message back to node B at time t13.However, node A receives the Verification Request message VREQ from nodeB at point t13, i.e. prior to receiving the Change CH message from nodeC which takes place later at time t14. Node A is therefore still in itsstate of disabling the verification procedure, not yet having ensuredthat all nodes are aware of the new ownership distribution, and willtherefore send no reply to the Verification Request message VREQ fromnode B.

[0059] As node B never receives any Verification Reply message from nodeA, it will continue to consider the status of the new slots as unsureand will hence not start using the new slots. Instead, it waits for apredefined period of time, at the end of which (at time t15), it onceagain initiates a verifying procedure with respect to the new slots, bysending out new Verification Request messages. When receiving therequest, at time t16), node A will have had time to receive theacknowledging Change CH message from node C (at time t14), and will nolonger disable the verification procedure. Hence, both node A and node Cnow responds to the request by sending their Verification Reply messagesVREP to reach node B at time t17. If it is assumed that neither of thesereplies indicate that A or C is using the slots that the VerificationRequest referred to, node B may now regard the new slots as free and mayaccordingly allocate them for use as desired.

[0060] Continuing with reference to FIG. 4, it is now assumed that afourth node D is connected to the link. This is detected by theoperation of a Link State Protocol at nodes C and D at time t21, at nodeB at time t24 and at node A at time t25, at which points in time therespective node will disable the verification procedure, as indicated bythe start of the patterned columns along the respective nodes' timeline.

[0061] Node B, for one reason or another, initiates a verificationprocedure for a specific set of slots at time t24, prior to becomingaware of the new node D at time t24, and sends Verification Requestmessages VREQ to nodes A and C (and not to node D as it is so farunaware of the existence of node D). However, when node C receives thismessage at time t23, it is already disabling the verification procedureas it has become aware of the new node at time t21 and consequentlysends no reply to the verification message to node B, rendering theverification uncompleted at node B, as has been discussed above. Whennode B subsequently, at time t24, becomes aware of the new node, it alsodisables the verification procedure and stops sending VerificationRequest messages.

[0062] As stated, node A becomes aware of the new node D at time t25 andsends a Change message CH containing the new list of nodes connected tothe link and including any changes in ownership distribution. Thismessage is received at node B at time t26, to be forwarded to node C andto subsequently reach node D at time t27. When this Change messagereaches node B, C, and D, they will conclude that it is consistent withthe new link data that their respective instances of the Link StateProtocol have provided, and will thus accept and forward the Changemessage, and at the same time stop their disabling of the verificationprocedure, as indicated by the bottom end of the three patterned columnsalong the timelines of node B, C and D. However, the verificationprocedure will not become fully enabled until the Change message CH onceagain reaches the master node A at time t30 and the master can concludethat all nodes are aware of the new link information and thus stopdisabling the verification procedure. Accordingly, when prior to that,node B, having stopped its disabling of the verification procedure whenreceiving the Change message CH at time t26, once again, at t28, triesto initiate a verification for the above mentioned set of slots bysending Verification Request messages to node A, C and D, thisverification will be blocked by node A.

[0063] As node B only receives Verification Reply messages from nodes Cand D but not from node A, it will continue to consider the status ofthe set of slots as unsure and will hence refrain form regarding thesubject set of slots as free for use. Instead, it waits for a predefinedperiod of time, at the end of which (at time t31), it once againinitiates a verifying procedure with respect to the set of slots, bysending out new Verification Request messages to nodes A, C and D. Whenreceiving the request, at time t32), node A will have received theacknowledging Change CH message from node D (at time t30), and will nolonger disable the verification procedure. Hence, both node A, C and Dnow responds to the request by sending their Verification Reply messagesVREP, the last thereof to reach node B at time t34. If it is assumedthat neither of these replies indicate that A, C or D is using the slotsthat the Verification Request referred to, node B may now regard the newslots as free and may accordingly allocate them for use as desired.

[0064] To be noted, when referring in this document to message beingexchanged between nodes connected to the link, that may favorably beimplemented as processes being performed by, and messages beingexchanged by, the nodes' interfaces to the subject link, as each nodemay have separate processes in operation with respect to separate linksthat the node has interfaces to.

[0065] Even though the invention has been exemplified above usingembodiments wherein a verifying feature according to the invention isused primarily for verifying or monitoring a conflict free write accesssituation, and/or a conflict free slot ownership distribution, bothbeing preferred uses, it may just as well be used to verify any othertype of slot/token access status.

[0066] Also, even though the invention has been described usingembodiment wherein a verifying feature according to the invention isused with respect to access to one or more slots, it may just asadvantageously be used to in systems wherein access to a slot or set ofslots may be limited to a portion of a link, thereby making it possiblefor two or more link interfaces to use the time slot on separateportions of the link. The inquiries and replies related to a verifyingprocedure according to the invention would then preferably includeoptional information on which portion of a link over which a linkinterface considers itself to have write access to the subject slot (orslots).

[0067] As is understood, many different alterations and modificationswith respect to embodiments described above, as realized by thoseskilled in the art, may be made within the scope of the invention, whichis defined by the accompanying claims.

1. A method in a communication network having a number of nodesconnected to the same link, the capacity of the link being divided intoframes which in turn are divided into time slots, comprising: aprocedure for informing each node on the link of which nodes that areconnected to the link and which slots that it has access to; a verifyingprocedure for subsequently verifying that information separately held bysaid nodes is not inconsistent regarding the nodes' right to allowsending of data in said slots; and the step of disabling said verifyingprocedure from producing a positive verification during transitionperiods at which different nodes on the link risk having inconsistentlink data.
 2. A method according to claim 1, said transition periodsstarting each time when determining that new link data is present andending each time when determining that other nodes on the link hasbecome aware of said new link data.
 3. A method according to claim 1,said disabling being performed during transition periods at which newlink data occurs, said link data being of a type that different nodesparticipating in said verifying procedure use as a basis for determiningtheir input to said verifying procedure.
 4. A method according to claim1, said transition periods including periods at which new link dataoccurs defining which nodes that are connected to the link.
 5. A methodaccording to claim 1, said transition periods including periods at whichnew link data occurs defining which nodes that have ownership of whichslots, said ownership referring to the respective node's right to allowsending of data in said set of slots.
 6. A method according to claim 1,said verifying procedure comprising a node on the link sending a requestmessage to the other nodes on the link asking for verification referringto said other nodes' information on their access to a slot, and each ofsaid other nodes responding thereto by sending a reply message to said anode informative of their respective information on their access to saidslot.
 7. A method according to claim 6, said verifying procedurecomprising a node on the link sending a request message to all othernodes on the link asking for verification referring to said other nodes'information on their access to a slot, and each of said other nodesresponding thereto by sending a reply message to said a node informativeof their respective information on their access to said slot.
 8. Amethod according to claim 6, said disabling of said verifying procedureincluding at least one node receiving such a request message refrainingfrom sending such a reply message in response thereto.
 9. A methodaccording to claim 6, said disabling of said verifying procedureincluding at least one node receiving such a request message refrainingfrom sending such a reply message in response thereto, to the extentsaid reply message indicates no obstacle for the node that sent therequest message to put said slot into use.
 10. A method according toclaim 6, said disabling of said verifying procedure including one ormore nodes refraining from sending such request messages to other nodeson the link.
 11. A method according to claim 1, wherein said nodes areassigned ownership of a respective set of slots, said ownershipreferring to the respective node's right to allow sending of data insaid set of slots.
 12. A method according to claim 11, said verificationprocedure being initiated repeatedly by a node with respect to all slotsit has been assigned ownership of to verify that no other node considersitself to have access to said slots, such a verification being aprerequisite for allowing sending of data in said slot.
 13. A methodaccording to claim 11, said verification procedure being initiated atleast once by a node after each change in what slots the node isassigned ownership of with respect to all slots that it has beenassigned ownership of to verify that no other node considers itself tohave access to said slots, such a verification being a prerequisite forallowing sending of data in said slot.
 14. A method according to claim11, said verification procedure being initiated at least once by a nodeafter each change in what slots the node is assigned ownership of withrespect to all new slots that it has been assigned ownership of and didwas not assigned ownership of prior to said change, to verify that noother node considers itself to have access to the new slots, such averification being a prerequisite for allowing sending of data in saidslot.
 15. A method according to claim 11, wherein control of theallocation of write access to a slot includes the right to allocatedsaid slot for use by a communication channel that is handled by the nodeas well as the right to the temporary allow another one of said nodes touse said slot.
 16. A method according to claim 15, said verificationprocedure being initiated repeatedly by a node with respect to all slotthat it is assigned ownership of and that it has temporarily other onesof said nodes to use, to verify whether or said other ones of said nodeare still considering themselves allowed to use said slots.
 17. A methodaccording to claim 16, said a node regarding a slot, which it hasallowed another node to use, as free for said a node to again allow foruse as desired, if the result of said verification procedure state thatno node on the link other than the said a node considers itself allowedto use said slot.
 18. A method according to claim 1, said verificationprocedure being used to verify that no two or more nodes regardthemselves as free to use, or put into use, the same time slot forsending data.
 19. A method according to claim 1, said disablingincluding each individual node on said link also, when disabling saidverifying procedure for said link, disabling any corresponding verifyingprocedures that the node participates in with respect to other linksthat the node is connected to.
 20. A method according to claim 1, saiddisabling comprising each one of said nodes disabling said verifyingprocedure from producing a positive verification when determining thatnew link data is present and discontinuing its disabling when receivinga link data message confirming other the new link data.
 21. A methodaccording to claim 1, said disabling comprising: each one of said nodesdisabling said verifying procedure from producing a positiveverification when determining that new link data is present, anddiscontinuing its disabling when receiving a link data message,originating from a master node on the link, confirming the new data; andsaid master node disabling said verifying procedure from producing apositive verification when determining that said new link data ispresent, transmitting said link data message confirming the new data toother nodes on the link, and discontinuing its disabling when receivingacknowledgement that all nodes have received said link data message. 22.A method according to claim 21, said link data message originating fromsaid master node and being circulated among the nodes connected to saidlink until it again reaches the master node, thus forming saidacknowledgement.
 23. A method according to claim 22, each noderefraining from forwarding said link data message if the informationcontained therein is inconsistent with the node's own link information.24. A method according to claim 22, each node, if receiving said linkdata message and finding the information contained therein to be isinconsistent with the node's own link information, sending aninconsistency message informative of the inconsistency to the masternode.
 25. A method according to claim 24, said master node, if receivingsuch an inconsistency message, resending a new link data messageincluding link data that have been modified to eliminate the detectedinconsistency.